The present invention is directed to the field of antennas used for RF data communications devices, particularly those used to transmit and receive digital signals, e.g., two-way pagers and the like. The antennas used with previous RF data communications devices are prone to significant problems. Many previous pagers are "one-way" pagers that are only able to receive a pager signal. However, many factors can contribute to the loss of an incoming message signal. Thus, it is desirable to employ a "two-way" pager that sends an acknowledgment signal to the remote station to confirm receipt of a message or to originate a message.
In previous VHF one-way pagers, it had been common to use a loop-type antenna, which is effective at receiving signals in the presence of the human body, which has properties that tend to enhance VHF radio signals. However, loop-type antennas are poor at the UHF frequencies needed for two-way pagers. Also, such antennas are typically embedded in a dielectric plastic pager body, which reduces the effective bandwidth of the received signal. Such a configuration has a very narrow bandwidth of typically about 1%. Such antennas also have poor gain performance when transmitting a signal, and are thus not useful for a two-way pager design.
Many previous two-way telecommunications devices use a "patch" antenna, in which a large, flat conducting member is used for sending and receiving signals. Patch antennas permit two-way communication under certain narrow bandwidth conditions, but do not provide a desirable radiation pattern. Signals propagate perpendicular to the flat surfaces of the antenna, and so the acknowledgment signal diverges within a bi-lobed conical envelope along an axis of propagation. While the signal transmits well "in front" and "behind" the pager, performance is poor if the signal axis is not well aligned with the remote station. Also, patch antennas are large, and can be as large as 16.times.16 cm.sup.2. While this may be fine for a mobile laptop computer, such is not well suited for a small hand-held mobile unit such as a pager. Patch antennas can be made smaller, but at a significant sacrifice of gain.
An improved two-way pager antenna design is shown in U.S. Ser. No. 08/715,347, filed Sep. 18, 1996, entitled "Antenna System For An RF Data Communications Device." This design incorporates a dipole antenna capable of sending and receiving signals having both vertical and horizontal polarization components, thereby increasing the likelihood of acquiring the signal. The dipole antenna is incorporated into the pager lid and anisotropically coupled to the LCD pager display element. This coupling effect divides the central frequency into two separate peaks, thereby increasing pager bandwidth.
While excellent under ideal conditions, the coupling effect varies as a function of the spatial distance separating the LCD, variations in the anisotropic composition of the LCD, and ground planes of the pager circuit boards. As the lid is opened and closed, antenna gain can vary between 0 to 1 dB and -1 to 0 dB. Also, as this distance varies, the center frequency changes, affecting the antenna's very wide bandwidth. These effects tend to degrade antenna performance in either send or receive modes.
The above-noted design incorporates a RF switch to change the antenna between transmit and receive modes. This switch is expensive and very fragile to electrostatic discharge, adding expense to the manufacture and maintenance of the unit. Also, this switch is lossy, reducing antenna gain by about 0.5 dB. Further, with this design, LCD placement with respect to the antenna is critical, requiring fine tuning and tight manufacturing tolerances, resulting in labor-intensive (and thus expensive) manufacturing. Also, with the previous antenna design, impedance matching with the radio circuit is difficult. Testing the previous antenna is difficult since it could only be tested in an assembled pager, and so antenna failures contribute to unit failures during testing. Also, the antenna tends to interfere with the radio components in the pager, thereby further reducing performance.